Fig. 3: Nutrient regulation of relative fluorescence yields.

a,b, Elevated \({F/{\rm{Chl}}}_{{\rm{active}}}\) at the high-nitrate, Fe-limited sites, which was reduced following Fe supply. In a, bars are means of all experimental treatments with or without added Fe for the Fe-limited sites 2 and 3 (left), and the predominantly N-limited sites 4 and 5 (right). Dots show individual data points, and error bars indicate the standard deviation, with two-sided, unpaired t-test P values for significantly different means indicated (n = 30 biologically independent samples for –Fe and n = 36 for +Fe). In b, \({F/{\rm{Chl}}}_{{\rm{active}}}\) is for night-time fluorescence data alone (points indicate the mean and error bars show the standard deviation; n = 5–7 biologically independent samples). c, \({F/{\rm{Chl}}}_{{\rm{passive}}}^{{\rm{ship}}}\)–photosynthetically active radiation (PAR) response curves from the shipboard radiometry. Red lines are for the Fe-limited zone (sites 1–3) and blue lines are for the predominantly N-limited zone (sites 4 and 5); bold symbols and lines indicate PAR bin averages with model fits (\({F/{\rm{Chl}}}_{{\rm{passive}}}^{{\rm{ship}}}=a\times \tanh [b\times {\rm{PAR}}/a]\), where a and b parameterize the light-saturated plateau and light-limited slope, respectively; ref. ⁴²). d, Changes in \({F/{\rm{Chl}}}_{{\rm{passive}}}\) across the Pacific Ocean transect from the shipboard radiometry and a boreal winter climatological average from the MODIS-Aqua sensor (satellite (sat)). Black dots show individual radiometric observations between 12:00 and 15:00 local time, and open circles and error bars show the mean and standard deviation (n = 36 independent radiometric observations). Days with the same letter labels have indistinguishable mean \({F/{\rm{Chl}}}_{{\rm{passive}}}^{{\rm{ship}}}\) values (one-way ANOVA, α < 0.05, followed by Tukey’s post hoc test). For all panels, units of \({F/{\rm{Chl}}}_{{\rm{passive}}}\) are Wm⁻² sr⁻¹ μm⁻¹ [mg Chl m⁻³]⁻¹.